Compleximetric Titration

Various compounds or substances can be used in quantitative analysis, including complexes. A complex or coordination compound is a molecular entity composed of a central metal ion reacting with electron-pair donors called ligands. The ligands must have at least one pair of unshared electrons available to form a covalent bond. They can be grouped in different ways depending on the number of coordination bonds they manage to form with the transition metal. Monodentate forms only one coordination bond, while polydentate forms more bonds. Complexes that polydentate ligands form are called chelates.

A certain form of volumetric analysis called complexometric titration uses complexes. In this analysis, the formation of a colored complex is used to indicate the end point of a titration. Complexometric titration is useful in determining mixture of different metal ions in solution. Because of polydentate ligands, it is easier to form complexes related to monodentate ligands, and chelates with higher stability can be formed. This can be used as a volumetric technique as long as the reaction reaches equilibrium rapidly after each portion of titrant is added, interfering situations do not arise, and a complexometric indicator capable of locating equivalence point with fair accuracy is available.

Metal ions can be titrated by polydentate ligands that form chelates which are very stable and water-soluble and have 1:1 ratio, also known as chelons, given by the reaction below:

Mn+ + L: (equilibrium arrow) (M-L)m+

where M is the metal ion, L is the chelon, the superscripts are the charges of the ions, and the product formed is the chelate.

The most common complexometric titrant is EDTA, an abbreviation for ethylenediaminetetraacetic acid and is represented as H4Y. EDTA is a polycarboxylic acid, an organic compound containing two or more carboxylic groups (-COOH). Deprotonation happens on the oxygens in the four carboxylic groups, and one lone electron pair left from the deprotonated oxygens can form a coordination bond with the metal ion. Also, the nitrogen atoms also have a lone electron pair each, also available to form a coordination bond with the metal.

(insert structure of EDTA here)

EDTA is a useful chelon and is very common since it has properties such as:

1. Forming stable water-soluble, 1:1 complexes with various metal ions such as Ba2+, Ca2+, Cd2+, Fe2+, Fe3+, Hg2+, Mg2+, Mn2+, Ni2+, and Sr2+;
2. Depending on pH on the formation of strong and stable complexes;
3. Being a primary standard;
4. Having no occurrence of coprecipitation due to soluble complexes; and
5. Having an equivalence point that can be determined in various ways.

In the case of EDTA titrations, the solution is usually standardized with calcium carbonate, especially in titrating Ca2+ and Mg2+.  The reaction involved when calcium carbonate is dissolved in acid is:

CaCO3 + 2H3O+ (equilibrium arrow) Ca2+ + CO2 + 3H2O

The indicators used in these titrations are organic colorants that form colored chelates with metal ions. These indicators are known as metal ion or metallochromic indicators. In order to indicate the endpoint, a metal ion indicator must be less stable than the EDTA complex itself. Examples of indicators in titrating Ca2+ and Mg2+ with EDTA are Arsenazo, Calmagite, and Eriochrome Black T (EBT). Other metal ion indicators include Murexide, Xylenol orange, and Pyrocatechol violet.